xep-0438.xml

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<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY OWASP "the <span class='ref'><link url='https://owasp.org/'>Open Web Application Security Project (OWASP)</link></span> <note>The Open Web Application Security Project (OWASP, or OWASP Foundation) is a nonprofit foundation that works to improve the security of software. For further information, see &lt;<link url='https://owasp.org/'>https://owasp.org/</link>&gt;.</note>" >
  <!ENTITY owasppasswords "<span class='ref'><link url='https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html'>OWASP Password Storage Cheat Sheet</link></span> <note>OWASP Cheat Sheet Series for password storage &lt;<link url='https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html'>https://cheatsheetseries.owasp.org/cheatsheets/Password_Storage_Cheat_Sheet.html</link>&gt;.</note>" >
  <!ENTITY rfc2195 "<span class='ref'><link url='http://tools.ietf.org/html/rfc2195'>RFC 2195</link></span> <note>RFC 2195: IMAP/POP AUTHorize Extension for Simple Challenge/Response &lt;<link url='http://tools.ietf.org/html/rfc2195'>http://tools.ietf.org/html/rfc2195</link>&gt;.</note>" >
  <!ENTITY rfc5746 "<span class='ref'><link url='http://tools.ietf.org/html/rfc5746'>RFC 5746</link></span> <note>RFC 5746: Transport Layer Security (TLS) Renegotiation Indication Extension &lt;<link url='http://tools.ietf.org/html/rfc5746'>http://tools.ietf.org/html/rfc5746</link>&gt;.</note>" >
  <!ENTITY rfc7677 "<span class='ref'><link url='http://tools.ietf.org/html/rfc7677'>RFC 7677</link></span> <note>RFC 7677: SCRAM-SHA-256 and SCRAM-SHA-256-PLUS Simple Authentication and Security Layer (SASL) Mechanisms &lt;<link url='http://tools.ietf.org/html/rfc7677'>http://tools.ietf.org/html/rfc7677</link>&gt;.</note>" >
  <!ENTITY rfc8018 "<span class='ref'><link url='http://tools.ietf.org/html/rfc8018'>RFC 8018</link></span> <note>RFC 8018: PKCS #5: Password-Based Cryptography Specification Version 2.1 &lt;<link url='http://tools.ietf.org/html/rfc8018'>http://tools.ietf.org/html/rfc8018</link>&gt;.</note>" >
  <!ENTITY rfc8265 "<span class='ref'><link url='http://tools.ietf.org/html/rfc8265'>RFC 8265</link></span> <note>RFC 8265: Preparation, Enforcement, and Comparison of Internationalized Strings Representing Usernames and Passwords &lt;<link url='http://tools.ietf.org/html/rfc8265'>http://tools.ietf.org/html/rfc8265</link>&gt;.</note>" >
  <!ENTITY nistsp800-63-3 "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-63-3'>Digital Identity Guidelines</link></span> <note>Digital Identity Guidelines, NIST Special Publication 800-63-3 &lt;<link url='https://doi.org/10.6028/NIST.SP.800-63-3'>https://doi.org/10.6028/NIST.SP.800-63-3</link>&gt;.</note>" >
  <!ENTITY nistsp800-63b "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-63b'>Digital Identity Guidelines: Authentication and Lifecycle Management</link></span> <note>Digital Identity Guidelines: Authentication and Lifecycle Management, NIST Special Publication 800-63B &lt;<link url='https://doi.org/10.6028/NIST.SP.800-63b'>https://doi.org/10.6028/NIST.SP.800-63b</link>&gt;.</note>" >
  <!ENTITY nistsp800-132 "<span class='ref'><link url='https://doi.org/10.6028/NIST.SP.800-132'>Recommendation for Password-Based Key Derivation, Part 1: Storage Applications</link></span> <note>Recommendation for Password-Based Key Derivation, Part 1: Storage Applications, NIST Special Publication 800-132 &lt;<link url='https://doi.org/10.6028/NIST.SP.800-132'>https://doi.org/10.6028/NIST.SP.800-132</link>&gt;.</note>" >
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Best practices for password hashing and storage</title>
  <abstract>
    This document outlines best practices for handling user passwords on the
    public Jabber network for both clients and servers.
  </abstract>
  &LEGALNOTICE;
  <number>0438</number>
  <status>Experimental</status>
  <type>Informational</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies/>
  <supersedes/>
  <supersededby/>
  <shortname>passwords</shortname>
  &sam;
  <revision>
    <version>0.2.0</version>
    <date>2020-10-30</date>
    <initials>ssw</initials>
    <remark><p>Update to match draft-ietf-kitten-password-storage-01.</p></remark>
  </revision>
  <revision>
    <version>0.1.1</version>
    <date>2020-05-05</date>
    <initials>ssw</initials>
    <remark><p>Fix reference to external document.</p></remark>
  </revision>
  <revision>
    <version>0.1.0</version>
    <date>2020-05-05</date>
    <initials>XEP Editor (jsc)</initials>
    <remark>Accepted by vote of Council on 2020-04-22.</remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2020-04-19</date>
    <initials>ssw</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>
<section1 topic='Introduction' anchor='intro'>
  <p>
    Following best practices when hashing and storing passwords and other
    authenticator secrets impacts a great deal more than just a users identity.
    It also effects usability, and backwards compatibility by determining what
    authentication and authorization mechanisms can be used.
    Unfortunately, aside from mandating the use of SCRAM-SHA-1 in &rfc6120;, and
    recommending at least 4096 rounds of PBKDF2 in &rfc5802; (a
    number which is now woefully inadequate), no general recommendations for
    best practices in password storage, transmission, or key derivation function
    tuning exist in the XMPP ecosystem.
  </p>
  <p>
    Many of the recommendations in this document were taken from
    &nistsp800-63b; and &nistsp800-132;.
  </p>
</section1>
<section1 topic='Requirements' anchor='reqs'>
  <p>
    This document makes specific recommendations for best practices on the
    public Jabber network for both clients and servers.
    It does not attempt to address private networks or proprietary services
    which may have different requirements, use cases, and security models.
    These recommendations include the hashing and storage of memorized secrets
    and other authenticators, authentication, and compatibility between clients
    and servers with respect to authentication.
  </p>
  <p>
    To keep the length of this document manageable, we assume basic familiarity
    with password storage and handling, common terms, and cryptographic
    operations.
    For an overview of basic password security see the &owasppasswords;
    maintained by &OWASP;.
  </p>
</section1>
<section1 topic='Glossary' anchor='glossary'>
  <p>
    Various security-related terms are to be understood in the sense
    defined in &rfc4949;
    Some may also be defined in defined in &nistsp800-63-3; Appendix A.1 and in
    &nistsp800-132; §3.1.
  </p>
  <p>
    Throughout this document the term "password" is used to mean any password,
    passphrase, PIN, or other memorized secret.
  </p>
  <p>
    Other common terms used throughout this document include:
  </p>
  <dl>
    <di>
      <dt>Mechanism Pinning</dt>
      <dd>
        Mechanism pinning A security mechanism which allows SASL clients to
        resist downgrade attacks.
        Clients that implement mechanism pinning remember the perceived strength
        of the SASL mechanism used in a previous successful authentication
        attempt and thereafter only authenticate using mechanisms of equal or
        higher perceived strength.
      </dd>
    </di>
    <di>
      <dt>Pepper</dt>
      <dd>
        A secret added to a password hash like a salt.
        Unlike a salt, peppers are secret and not unique.
        They must not be stored alongside the hashed password.
      </dd>
    </di>
    <di>
      <dt>Salt</dt>
      <dd>
        In this document salt is used as defined in &rfc4949;.
      </dd>
    </di>
  </dl>
</section1>
<section1 topic='SASL Mechanisms' anchor='required'>
  <p>
    Clients and servers must already implement the SASL mechanisms listed in RFC
    6120 §13.8.1 For Authentication Only.
    These mechanisms are:
  </p>
  <ul>
    <li>SCRAM-SHA-1</li>
    <li>SCRAM-SHA-1-PLUS</li>
  </ul>
  <p>
    In addition, clients and servers SHOULD support the following SCRAM variants
    defined in &rfc7677;:
  </p>
  <ul>
    <li>SCRAM-SHA-256</li>
    <li>SCRAM-SHA-256-PLUS</li>
  </ul>
  <p>
    Clients SHOULD NOT invent their own mechanisms that have not been
    standardized by the IETF, the XSF, or another reputable standards body.
  </p>
  <p>
    Clients MUST NOT implement any mechanism with a usage status of "OBSOLETE",
    "MUST NOT be used", or "LIMITED" in the &ianasasl;.
    Similarly, any mechanism that depends on a hash function listed as "MUST
    NOT" in &xep0414; MUST NOT be used.
    This means that the following mechanisms which were commonly used with XMPP
    in the past MUST NOT be supported:
  </p>
  <ul>
    <li>CRAM-MD5 (&rfc2195;)</li>
    <li>DIGEST-MD5 (&rfc6331;)</li>
  </ul>
</section1>
<section1 topic='Client Best Practices' anchor='client'>
  <section2 topic='Mechanism Pinning' anchor='pinning'>
    <p>
      Clients maintain a list of preferred SASL mechanisms, generally ordered by
      perceived strength to enable strong authentication (&rfc6120; §6.3.3
      Mechanism Preferences).
      To prevent downgrade attacks by a malicious actor that has successfully
      man in the middled a connection, or compromised a trusted server's
      configuration, clients SHOULD implement "mechanism pinning".
      That is, after the first successful authentication with a strong
      mechanism, clients SHOULD make a record of the authentication and
      thereafter only advertise and use mechanisms of equal or higher perceived
      strength.
    </p>
    <p>
      For reference, the following mechanisms are ordered by their perceived
      strength from strongest to weakest with mechanisms of equal strength on
      the same line.
      This list is a non-normative example and does not indicate that these
      mechanisms should or should not be supported:
    </p>
    <ol>
      <li>EXTERNAL</li>
      <li>SCRAM-SHA-1-PLUS, SCRAM-SHA-256-PLUS</li>
      <li>SCRAM-SHA-1, SCRAM-SHA-256</li>
      <li>PLAIN</li>
      <li>DIGEST-MD5, CRAM-MD5</li>
    </ol>
    <p>
      The EXTERNAL mechanism defined in &rfc4422; appendix A is placed at
      the top of the list.
      However, its perceived strength depends on the underlying authentication
      protocol.
      In this example, we assume that TLS (&rfc8446;) services are being used.
    </p>
    <p>
      The channel binding ("-PLUS") variants of SCRAM (&rfc5802;) are listed
      above their non-channel binding cousins, but may not always be
      available depending on the type of channel binding data available to
      the SASL negotiator.
    </p>
    <p>
      The PLAIN mechanism sends the username and password in plain text,
      but does allow for the use of a strong key derivation function (KDF)
      for the stored version of the password on the server.
    </p>
    <p>
      Finally, the DIGEST-MD5 and CRAM-MD5 mechanisms are listed last
      because they use weak hashes and ciphers and prevent the server from
      storing passwords using a KDF.  For a list of problems with DIGEST-
      MD5 see &rfc6331;.
    </p>
  </section2>
  <section2 topic='Storage' anchor='client-storage'>
    <p>
      Clients SHOULD always store authenticators in a trusted and encrypted
      keystore such as the system keystore, or an encrypted store created
      specifically for the clients use.
      They SHOULD NOT store authenticators as plain text.
    </p>
    <p>
      If clients know that they will only ever authenticate using a mechanism
      such as SCRAM where the original password is not needed (for example if
      the mechanism has been pinned) they SHOULD store the SCRAM bits or the
      hashed and salted password instead of the original password.
      However, if backwards compatibility with servers that only support the
      PLAIN mechanism or other mechanisms that require using the original
      password is required, clients MAY choose to store the original password
      so long as an appropriate keystore is used.
    </p>
  </section2>
</section1>
<section1 topic='Server Best Practices' anchor='server'>
  <section2 topic='Additional SASL Requirements' anchor='server-required'>
    <p>
      Servers MUST NOT support any mechanism that would require authenticators
      to be stored in such a way that they could be recovered in plain text from
      the stored information.
      This includes mechanisms that store authenticators using reversable
      encryption, obsolete hashing mechanisms such as MD5, and hashes that are
      unsuitable for use with authenticators such as SHA256.
    </p>
  </section2>
  <section2 topic='Storage' anchor='server-storage'>
    <p>
      Servers MUST always store passwords only after they have been salted and
      hashed using a strong KDF.
      If multiple hashes are supported for use with SCRAM, for example
      SCRAM-SHA-1 and SCRAM-SHA-256, separate salted and hashed passwords SHOULD
      be calculated and stored for each mechanism so that users can log in with
      multiple clients that support only some of the mechanisms.
    </p>
    <p>
      A distinct salt SHOULD be used for each user, and each SCRAM family
      supported.
      Salts MUST be generated using a cryptographically secure random number
      generator.
      The salt MAY be stored in the same datastore as the password.
      If it is stored alongside the password, it SHOULD be combined with a
      pepper stored in the application configuration, an environment variable,
      or some other location other than the datastore containing the salts.
    </p>
    <p>
      The following minimum restrictions MUST be observed when generating salts
      and peppers.
      More up to date numbers may be found in &owasppasswords;
    </p>
    <dl>
      <di>
        <dt>Minimum Salt Length</dt>
        <dd>
          16 bytes
        </dd>
      </di>
      <di>
        <dt>Minimum Pepper Length</dt>
        <dd>
          32 bytes
        </dd>
      </di>
    </dl>
  </section2>
  <section2 topic='Authentication and Rotation' anchor='auth'>
    <p>
      When authenticating using PLAIN or similar mechanisms that involve
      transmitting the original password to the server the password MUST
      be hashed and compared against the salted and hashed password in the
      database using a constant time comparison.
    </p>
    <p>
      Each time a password is changed or reset, a new random salt should be
      created and the iteration count and pepper (if applicable) should be
      updated to the latest value required by server policy.
    </p>
    <p>
      If a pepper is used, consideration should be taken to ensure that it can
      be easily rotated.
      For example, multiple peppers could be stored with new passwords and
      reset passwords using the latest pepper.
      A hash of the pepper using a cryptographically secure hash function such
      as SHA256 could then be stored in the database next to the salt so that
      future logins can identify which pepper in the list was used.
      This is just one example, pepper rotation schemes are outside the scope of
      this document.
    </p>
  </section2>
</section1>
<section1 topic='PBKDF2 Parameters' anchor='pbkdf2'>
  <p>
    Because the PBKDF2 key derivation function (&rfc8018;) is used by
    SCRAM-SHA-1 which is mandated for use in XMPP, this document recommends it
    for password storage.
    Servers SHOULD use the following parameters when applying PBKDF2:
  </p>
  <dl>
    <di>
      <dt>Minimum iteration count (c)</dt>
      <dd>10,000 (100,000 for higher security environments)</dd>
    </di>
    <di>
      <dt>Hash</dt>
      <dd>SHA256</dd>
    </di>
    <di>
      <dt>Output length (dkLen)</dt>
      <dd>hLen (length of the chosen hash)</dd>
    </di>
  </dl>
  <p>
    The minimum iteration count may be tuned to the specific system on which
    password hashing is taking place.
  </p>
</section1>
<section1 topic='Password Complexity Requirements' anchor='passwordcomplexity'>
  <p>
    Before any other password complexity requirements are checked, the
    preparation and enforcement steps of the OpaqueString profile of &rfc8265;
    SHOULD be applied (for more information see the Internationalization
    Considerations section).
    Entities SHOULD enforce a minimum length of 8 characters for user passwords.
    If using a mechanism such as PLAIN where the server performs hashing on the
    original password, a maximum length between 64 and 128 characters MAY be
    imposed to prevent denial of service (DoS) attacks.
    Entities SHOULD NOT apply any other password restrictions.
  </p>
  <p>
    In addition to these password complexity requirements, servers SHOULD
    maintain a password blocklist and reject attempts by a claimant to use
    passwords on the blocklist during registration or password reset.
    The contents of this blocklist are a matter of server policy.
    Some common recommendations include lists of common passwords that are not
    otherwise prevented by length requirements, and passwords present in known
    breaches.
  </p>
</section1>
<section1 topic='Security Considerations' anchor='security'>
  <p>
    This document contains recommendations that are likely to change over time.
    It should be reviewed regularly to ensure that it remains accurate and up to
    date.
    Many of the recommendations in this document were taken from
    &owasppasswords;, &nistsp800-63b;, and &nistsp800-132;.
  </p>
  <p>
    The SCRAM suite of SASL mechanisms are recommended in this document,
    however, there is currently no way to force a password reset.
    This reduces upgrade agility if a weakness is discovered in SCRAM and means
    that new, untested, SCRAM-based or SCRAM-like mechanisms should be added
    with caution.
  </p>
  <p>
    The "-PLUS" variants of SCRAM support channel binding to their underlying
    security layer, but lack a mechanism for negotiating what type of channel
    binding to use.
    In &rfc5802; the tls-unique (&rfc5929;) channel
    binding mechanism is specified as the default, and it is therefore likely to
    be used in most applications that support channel binding.
    However, in the absence of the TLS extended master secret fix (&rfc7627;)
    and the renegotiation indication TLS extension (&rfc5746;) the tls-unique
    and tls-server-endpoint channel binding data can be forged by an attacker
    that can MITM the connection.
    Before advertising a channel binding SASL mechanism, entities MUST ensure
    that both the TLS extended master secret fix and the renegotiation
    indication extension are in place and that the connection has not been
    renegotiated.
  </p>
  <p>
    This document mentions many hash functions that are already in
    use in the XMPP ecosystem, or that have been used in the past.
    It does not make recommendations for what functions should or should not be
    used in new applications.
    For recommendations about the use of hash functions and their security
    implications, see &xep0414;
  </p>
  <p>
    For TLS 1.3 no channel binding types are currently defined. Channel binding
    SASL mechanisms MUST NOT be advertised or negotiated over a TLS 1.3 channel
    until such types are defined.
  </p>
</section1>
<section1 topic='Internationalization Considerations' anchor='security'>
  <p>
    The PRECIS framework (Preparation, Enforcement, and Comparison of
    Internationalized Strings) defined in &rfc8264; is used to enforce
    internationalization rules on strings and to prevent common application
    security issues arrising from allowing the full range of Unicode codepoints
    in usernames, passwords, and other identifiers.
    The OpaqueString profile of &rfc8265; is used in this document to ensure
    that codepoints in passwords are treated carefully and consistently.
    This ensures that users typing certain characters on different keyboards
    that may provide different versions of the same character will still be able
    to log in.
    For example, some keyboards may output the full-width version of a character
    while other keyboards output the half-width version of the same character.
    The Width Mapping rule of the OpaqueString profile addresses this and
    ensures that comparison succeeds and the claimant is able to be
    authenticated.
  </p>
</section1>
<section1 topic='IANA Considerations' anchor='iana'>
  <p>
    This document requires no interaction with &IANA;.
  </p>
</section1>
<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <p>
    No namespaces or parameters need to be registered with the &REGISTRAR; as a
    result of this document.
  </p>
</section1>
</xep>